fix(mcp-proxy): honor approval timeout across poll slices

[oleg-bk]

Summary

• keep nonblocking approval watchers alive until the configured approval timeout expires
• prevent approved retries from creating a second pending approval prompt
• add product-path regression tests for approval -> retry -> downstream execution

Scope

• In scope: MCP Proxy local approval manager and focused approval/runtime-gate tests.
• Out of scope: unknown-tool/schema-deny evidence, git/fetch policy, filesystem path rules, TrapDoor expansion, backend/private/control-plane/secrets, release/version bump.
• Public/private boundary: public SDK MCP Proxy package only; no backend/private logic added.

Product-Path Acceptance

• Product-path gate: pass
• Artifact fingerprint: branch codex/mcp-proxy-approval-retry-loop, commit e36cc5a, base 4ccc8ec
• Positive path: valid write call creates approval; after operator approval, identical retry executes downstream.
• Negative path: retry does not create a second pending approval prompt.
• Evidence path: parent approval reaches approved; child retry record reaches executed; no raw file content/secrets asserted in the focused acceptance path.
• Old-state / migration behavior: old evidence DB upgrade/recovery not touched; remains a separate issue.
• Remaining unverified: live Console with a real >60s human delay; deterministic test simulates the slow first poll slice without waiting 60s.

Type-specific gate:

• Approval path: request -> approval -> retry -> execution

Validation

• PYTHONPATH=../..:. python3 -m pytest tests/test_mcp_proxy_approval.py::test_nonblocking_watcher_honors_full_timeout_before_expiring tests/test_mcp_proxy_approval.py::test_nonblocking_retry_after_approval_executes_downstream tests/test_mcp_proxy_runtime_gate.py::test_waiting_retry_after_local_approval_executes_downstream -q -> 3 passed
• PYTHONPATH=../..:. python3 -m pytest tests/test_mcp_proxy_approval.py tests/test_mcp_proxy_runtime_gate.py tests/test_mcp_proxy_evidence.py -q -> 163 passed
• PYTHONPATH=../..:. python3 -m pytest tests/test_mcp_proxy_passthrough.py tests/test_mcp_proxy_passthrough_concurrent.py -q -> 68 passed, 1 skipped
• git diff --check / git diff --cached --check -> passed

Release / Claim Notes

• Claims this PR supports: approval retry can reuse a slow local approval and execute without a second pending prompt on the covered local approval/runtime-gate paths.
• Claims this PR does not support: unknown-tool/schema-deny evidence fixes, old evidence DB chain recovery, release readiness by itself.

Implemented with assistance from Codex.